Cold Expansion Method Research Articles

Effects of Cold Expansion on Residual Stress of 7050 Aluminium Alloy Frame Forging

Regulation of residual stress in a component is the key to improving its service performance. A cold expansion method was proposed for reducing the residual stress in 7050 aluminium alloy curved frame forging after quenching. The effect of the cold expansion method on the residual stress and equivalent plastic strain distribution of the 7050 aluminium alloy curved frame forging was investigated. The results showed that the maximum residual stress at the center thickness was reduced from 153 MPa to 94 MPa after the cold expansion, while it decreased from 283 MPa to 120 MPa at the surface with the highest stress reduction rate of 86.2%. The stress uniformity in the final forming region of the forging was improved. The equivalent plastic strain of the forging gradually decreases from the center to each side along the diameter of the expanded hole in cold expansion. The stress reduction effect matched with the distribution of equivalent plastic strain. The surface stress of the forging measured by x-rays diffraction (XRD) method was in agreement with the simulation results, and the reliability of the numerical model was verified. The cold expansion method can effectively reduce the quenched residual stress in curved frame forging.

Fatigue life enhancement mechanism and lifetime prediction of AA6061-T6 open-holed sheet treated by electromagnetic driving dynamic cold expansion

In this study, mechanisms of fatigue life enhancements of electromagnetic driving dynamic cold expansion (DCE) open holed sheets were investigated by testing fatigue life and obtaining residual stress fields experimentally and numerically. Furthermore, a fatigue life prediction method based on three-dimensional stress distributions was proposed. Results showed that DCE method extended higher fatigue life and produced more uniform residual stress along hole axial direction than static cold expansion (SCE) method. The differences in stress distribution led to different fatigue crack initiation and propagation modes of DCE and SCE specimens. Moreover, the predicted fatigue life results were in accordance with experimental data. • Dynamic cold expansion extended larger fatigue life for holed-sheets. • More uniform residual stress was induced by dynamic cold expansion. • Fatigue cracks propagated horizontally in dynamic cold expanded specimens. • Proposed method to predict fatigue life had good agreement with experiments.

Residual stress distribution and fatigue performance investigations of electromagnetic force-based dynamic cold expansion open-holed sheet

In this study, residual stress distributions and fatigue performances of electromagnetic driving dynamic cold expansion (DCE) open-holed sheets were investigated. Fatigue behaviors were tested, and residual stress fields were obtained experimentally and numerically. Furthermore, a fatigue life prediction method based on three-dimensional stress distribution was proposed. Results showed that DCE method extended higher fatigue life and produced more uniform residual stress along hole axial direction than static cold expansion (SCE) method. The stress distribution differences led to different fatigue crack initiation and propagation modes of DCE and SCE specimens. Uniform stress distributions along axial direction in DCE specimens contributed to random initiations and horizontal propagations of fatigue cracks, while the fatigue cracks in SCE specimens usually initiated at the mandrel entrance layer and propagated to the mandrel exit layer. Moreover, the predicted fatigue life results agreed well with experimental data.

Improvement in fatigue performance of thin fasteners via electromagnetic strengthening process

The fatigue behaviors of 1.5 mm-thick 2A12-T4 aluminum alloy fasteners processed by the electromagnetic cold expansion (EMCE) method are studied. The fatigue life of the fasteners processed by the EMCE method is 1.77 times that of the conventional direct mandrel cold expansion method at the maximum applied stress of 120 MPa. Finite element simulation shows that the EMCE process can introduce tensile stress in both hoop and radial directions, which avoids warping in thin fasteners. Moreover, the EMCE process achieves uniformly distributed compressive residual stress along the thickness direction, which is also beneficial to the fatigue improvement of the fasteners.

A dynamic cold expansion method to improve fatigue performance of holed structures based on electromagnetic load

In this study, a novel dynamic cold expansion (DCE) method based on electromagnetic load was developed to enhance the fatigue life of open hole structures. The experiments were conducted to investigate the enhance properties of DCE method, and the corresponding static cold expansion (SCE) method was also investigated as comparison. The expansion resistance was evaluated. The geometric dimension varieties of holes after cold expansion were measured. Moreover, the fatigue life of specimens after DCE and SCE were assessed, and the fatigue failure mechanism was discussed. Results showed that DCE method can obviously reduce expansion resistance compared with SCE method and generate more uniform hole diameter distribution along the hole axial direction after cold expansion. In fatigue tests, specimens worked by DCE exhibited the higher fatigue life than that worked by SCE, especially under high load level. Furthermore, DCE method was also observed to have significant effects on fatigue crack initiation and propagation of specimens.

A novel hole cold-expansion method and its effect on surface integrity of nickel-based superalloy

Preferred surface integrity around the hole wall is one of the key parameters to ensure the optimized performance of hole components for nickel-based superalloy. The novel hole cold expansion technique introduced in this work involves the laser texturing process (LTP) followed by the Hertz contact rotary expansion process (HCREP), where the cylindrical sleeve is the critical component connecting the above-mentioned two processes. The purpose of LTP is to obtain the most optimized strengthened cylindrical sleeve surface, preparing for the following HCREP. Hereafter, the HCREP acts on the nickel-based hole components by the rotary extruding movements of the strengthened sleeve and conical mandrel tools. As compared to the as-received GH4169 material, the surface integrity characterization for the strengthened hole shows that a plastic deformation layer with finer grains, higher micro-hardness, deeper compressive residual stress (CRS) distribution and lower surface roughness is formed at the hole wall. In addition, transmission electron microscope (TEM) observations reveal the microstructure evolution mechanism in the strengthened hole. Grain refinement near the hole wall is regarded as the fundamental reason for improving the surface integrity, where the aggregated dislocations and recombined dislocation walls can be clearly observed.

Fatigue Life Enhancement of a D16at Aluminum Alloy for Aircraft Components with Fastener Holes

D16AT aircraft Al-alloy was evaluated. The friction stir hole expansion (FSHE), solid mandrel cold working and symmetric cold expansion (SCE) methods were compared. The results are summarized on the basis of fatigue tests, S–N curves, X-ray diffraction, and microstructural analysis. Under the high-cycle fatigue, SCE provides more than 66 times longer fatigue life as compared to the solid mandrel method and more than 82 times greater fatigue life in comparison with the FSHE method. Through X-ray diffraction analysis it was found that the higher efficiency of the SCE method was due to symmetric distribution (with respect to the plate middle plane) of residual hoop stresses around the hole. On the other hand, the solid mandrel cold working method causes a significant gradient of the residual stress distribution through the plate thickness, which is a precondition for nucleation and propagation of corner fatigue cracks. The FSHE method efficiency was established to be primarily dependent on generated heat and equivalent plastic strains. The combination of these factors determines the beneficial microeffect of the microstructure modifying in the vicinity of the hole and a useful macroeffect due to residual compressive stresses. It was concluded that SCE method should be used for prestressing of fastener holes in the most loaded components in D16AT aircraft structures – wings and fuselage, while the FSHE one can be applied to processing of fastener holes in less loaded aircraft components.

Combined Cold Expansion and Friction Stir Processing of Fastener Holes in Aluminum Alloy Al-2014-T6

Cold expansion is used to induce compressive stress zone, by inserting a tapered mandrel/pin or ball into an undersized fastener hole in order to improve fatigue life. In this paper, combined cold expansion and friction stir processing of fastener holes in aluminium alloy Al-2014-T6 is presented wherein the cold expansion tool rotates and thus friction stirs the cylindrical surface of a fastener hole while expanding it. The residual stress, surface roughness and hardness of wall of fastener holes are measured and SEM images are captured. The combined cold expansion method is evaluated for three conditions at tool and fastener hole interface, namely wet (i.e., in presence of metal working fluid), solid lubricant (i.e., in presence of Al2O3 nanoparticles), and dry (i.e., without any external medium). The conventional cold expansion process is compared with combined cold expansion and friction stir processing. The results indicate efficacy of using nanoparticles as a medium in combined cold expansion and surface processing of Al-2014-T6 aluminium alloy to improve surface integrity and fatigue life.

Effect of Cold Expansion on High Cycle Fatigue of 7A85 Aluminum Alloy Straight Lugs

Abstract The cold expansion method was proposed to improve the high cycle fatigue (HCF) property of 7A85 aluminum alloy straight lugs. The microstructure, the microhardness and the residual stress near the hole wall of cold expanded lugs were investigated. The results indicate that a plastic deformation layer with about 150 μm in thickness is formed on the surface of the hole wall. Within the plastic deformation layer, the microhardness and the compressive residual stress are high, and the grains are stretched along the moving direction of the mandrel and compressed perpendicular to it. The fatigue life and fatigue strength were found to be improved significantly by cold expansion (CE). The fatigue crack initiation and propagation of cold expansion specimens were analyzed compared with those of the specimens without cold expansion (WCE). The mechanism of the improved fatigue property for cold expanded 7A85 aluminum alloy straight lugs was discussed.

Methods to detect and prevent fatigue in ageing aircraft structures

The paper is focused on the methods which detect and prevent material fatigue in ageing aircraft structures. The goal is to analyse material fatigue and its impact on aircraft structures. The significance and impact of fatigue is given in description of accidents that were caused directly by material fatigue of the aircraft structures. The most important and most commonly used methods to detect and prevent fatigue in aircraft structures are infinite lifetime concept methods, finite lifetime concept methods, non-destructive testing methods, cold expansion methods, built- in testing equipment, structural health monitoring methods and methods to detect corrosion fatigue.

Hole Cold Expansion the Fatigue Mitigation Game Changer of the Past 50 Years

Over the past 50 years, mitigating cracks from holes has been a major focus of aerospace research, design, and stress engineers. Technology and design philosophies were developed to focus on predicting and increasing fatigue life of metal aircraft structure. One of the innovations to have the biggest impact to slow or arrest crack growth was to induce beneficial residual stresses around a hole. The zone of residual compressive stresses shields the hole from the effects of cyclic loads. The split sleeve cold expansion method was pioneered in the early 1970s by Boeing and Fatigue Technology. The split sleeve cold expansion process has become the bench mark method in mitigating fatigue cracks and enhancing durability and damage tolerance of aerospace metal structures. This paper will review the history of cold expansion methods and the evolution of this technology to the controlled and widely accepted methods used by industry today.

Numerical Simulation on Stress Distribution and Failure of Composites Holes with Cold Expansion

A numerical investigation was conducted to apply cold expansion methods on composites structures under the guidance of cold expansion techniques of metal materials and interference-fit techniques of composites structures. Finite element expansion models with sleeves or without sleeves are established to study the stress distributions and failure conditions around cold expanded holes with different expansion values (0.5%, 1% and 1.5%). Results show that no damage occurs with expansion value 0.5% and using a sleeve in the expansion process can cause uniform stress distributions.

A novel method for improving fatigue life of friction stir spot welded joints using localized plasticity

This study introduces a new method for enhancing the life and strength of friction stir spot welded (FSSW) joints using a localized plasticity process. The aim of this investigation is to evaluate the effect of cold expansion on the improvement of fatigue life of FSSW joints in aluminum alloy 7075-T6 plates through conducting experimental and numerical procedures. In the experimental section, the fatigue tests were carried out using constant amplitude load control servo-hydraulic fatigue testing machine on the as-welded and cold expanded FSSW joints with cold expansion level of 3.3%. In the numerical part, three-dimensional finite element models were implemented to calculate stress and strain distributions in two batches of specimens using two multi-axial fatigue criteria: Smith–Watson–Topper and Fatemi–777Socie. The obtained experimental and numerical S–N data revealed that the cold expansion method could improve the fatigue life of FSSW joints in all load ranges so that in high cycle regimes, it could improve the fatigue life up to 6times. Although the results revealed that there is a relatively good agreement between estimated fatigue lives and experimental data in reasonable fatigue life regime, considering the fatigue crack growth life can lead to better estimation.

Experimental and numerical comparison of cold expansion and interference fit methods in improving fatigue life of holed plate in double shear lap joints

In this paper experimental and numerical investigations have been carried out on comparison of cold expansion and interference fit techniques in improving fatigue life of holed plates made from Al 2024-T3 in double shear lap joints. Fatigue tests were conducted on the specimens to study the role of cold expansion and interference fit on fatigue life and failure mode. Also to explain the experimental results and to obtain the residual stress and pre-stress distributions in the specimens due to these two techniques, finite element simulations were employed. The experimental results revealed that the interference fitted specimens exhibit much longer fatigue life, compared to cold expanded specimens at lower amplitude cyclic load, but as the load increases the two techniques show almost the same fatigue life. The finite element simulations show that interference fit reduces the local stress amplitude around the fastener hole, whereas cold expansion reduces the local mean stress. The numerical simulations can clarify and explain the experimentally observed results.

On the prediction of initiation life for fatigue crack emanating from small cold expanded holes

The cold expansion method is an effective method widely used for retarding the initiation of the fatigue crack. In the present work, the finite element analysis is performed to determine the residual stress distribution around the cold expanded hole and the ɛ−N method is implemented to predict the fatigue crack initiation lives. The superposition of the mean stress is used to analyze the fatigue crack initiation. It is found that the effect of Kf, which decreases the Kt effect, must be taken into account for the small stop-hole. Estimates of the fatigue life quantities correlate well with the experimental results.

Accuracy of Numerical Methods in Determination of Residual Stresses Due to Cold Expansion in Metallic Materials

Today more than ever, it was tried to reach the optimum usage of materials by applying various methods of manufacturing processes. Increasing the fatigue life of components is one of important aims. All parts of a machine must be connected together for incoming to service. Nonpermanent connections like bolt and nut joints must have hole on both components. Fatigue life was increased if these holes were reinforced by various methods. One of the ways is producing compressive residual stress in near fields of hole by cold expansion (CE) method. In this study the CE process was applied to plate was made from elastic–perfectly plastic material. Residual stresses distributions were obtained by analytical and numerical methods (using finite elements analysis). Results of these methods were compared together for determination the accuracy of numerical methods. In the next step, residual stress distribution due to the CE in real material (aluminum) was determined numerically. Three numerical methods were used: really simulation method (3D plate with punch), 2D and 3D model with internal pressure (using try and error, the amount of pressure that was needed to introduce the same CE ratio was determined) and finally, 2D and 3D model with radial displacement in internal surface of the hole. It was determined that the first and second methods have good agreement with experimental results so can be used for simulation of the CE process.

Hole Expansion by Using Al-Alloy Specimen and 0.45% Carbon Steel Specimen

The enhancement method of fatigue life and the crack initiate and growth behavior of a holed specimen was investigated by using the 2024 Aluminum alloy and 0.45% Carbon steel. The purpose of present study is to propose a simple technical method for enhancement of fatigue life in a notched specimen. Also, the effect of local plastic deformation by cold work on fatigue crack initiation behavior was examined. This paper presents a basic experimental kinematic cold expansion method by inserting and removing a pin through the specimen hole. The shape of cross-section of pin was a circle or an ellipse. It was shown that the fatigue life of the specimen with the cold-worked hole was longer than that of the specimen with non-cold-worked hole for the case of same stress level in aluminum alloy and carbon steel. Also, the fatigue strength was higher in the case of the cold expanded hole. In this study, a methodology of lengthening of fatigue life of holed specimen is shown. Also, the improvement conditions of fatigue life were significantly affected by shape of pin, local hardening and residual stress conditions. The fatigue life improvement of the damaged component of structures was studied.

The Effect of Cold Expansion on the Fatigue Life of the Chamfered Holes

The cold expansion method is one of the most popular techniques in the fatigue enhancement processes, and it has been widely used as a means of improving the fatigue resistance for aircraft structures with holes. Cold expanded holes have lower compressive residual stresses on the entry surface rather than the middle and exit surfaces. Due to the nonuniform residual stress distribution, fatigue crack initiation often occurs on the entry surface. This study proposes a new approach to increase the compressive residual stress magnitude at the entry of the hole. The new method is to apply chamfers into holes before the cold expansion process. Split mandrel process was used to cold work the hole with and without chamfers. Both numerical and experimental studies were done to verify the effects of hole chamfers on the residual stress distribution of the cold expanded holes. Finite element analysis (FEA) was conducted in order to see the effects of the chamfer geometries on the residual stress distributions. The FEA results showed an improvement of compressive residual stress magnitudes at the entry position of the cold expanded hole. The numerical results were compared with X-ray diffraction measurements. Fatigue tests were done to compare the fatigue life of the holes with various chamfer sizes and angles. The cold expansion chamfered holes showed a clear improvement in fatigue life over cold expanded holes without chamfers.

3-Dimensional Finite Element Analysis of the Residual Stress by Cold Expansion Method and Interference Fit

Cold expansion method and interference fit of fastener hole have been used for over 40 years by the aircraft industry to improve the fatigue life of structures because they induce compressive residual stresses around holes. Especially, interference fits are very widely applied in the industry, because of their simple manufacturing process. There have been only few studies on the difference in the effect between cold expansion method and interference fit. The purpose of this study is to compare the effect between cold expansion method and interference fit according to plate thickness. Furthermore, residual stress distribution according to real clamping force is also investigated.

Finite Element Analysis of the Stress Intensity Factor and the Residual Stress by Cold Expansion Method in CT Specimen

Cold expansion method retards the crack initiation due to the compressive residual stress developed on a hole surface. Most previous researches have shown only the beneficial distribution of residual stresses in the retardation of the crack initiation at the stress concentration area. Also, there have been only few studies on the relation between crack growth and residual stress around other adjacent holes. A few fastener holes of aircraft structures is a shot distance which is less than 20mm between holes. The purpose of this study is to provide better understanding of the residual stress effect around a hole in a structure as crack growth starts from another hole. By finite element method, this study showed that residual stress in a CT specimen is redistributed by cold expansion process and that tensile stress increases in proportion to the cold expansion ratio in the vicinity of the crack. Stress intensity factor increases as the cold expansion ratio increases.